There are many reasons for a wired communications device or a wireless communications device to change the power level being used to communicate with another device. Possible motivations for changing the power level may involve a desire to decrease the potential for interference with other devices or comply with regulatory standards specifying a certain power spectral density mask.
Interference among devices is a significant problem in both wireless communications and wired communications networks. In wired networks, the interference is partly due to the close proximity of cables next to each other carrying signals at high frequencies and often in a common band.
The problem of interference is particularly bad in wired networks that were designed to carry relatively low-frequency, low-power analog signals, such as voice telephone signals, but are now being used to carry relatively high frequency and higher power modem signals. FIG. 1a illustrates a bundle of cables carrying various types of signals as commonly found in a bundle including cables for telephone signals.
As illustrated in FIG. 1a, the increasing use of digital subscriber line (DSL) technology to provide relatively high-speed access to the Internet has made the problem of interference in cables carrying telephone and data signals more common and more likely. Examples of DSL technology include HDSL and ADSL. HDSL refers to high-data rate DSL, and ADSL refers to asymmetric DSL. In ADSL, the upstream data rate from the subscriber to the central office is lower than the downstream data rate from the central office to the subscriber. A particularly bad form of interference in the wired communications context and particularly for DSL communications is referred to as crosstalk.
FIG. 1b illustrates examples of crosstalk in DSL links. There are several types of crosstalk including two typically dominant ones: near-end crosstalk (NE-XT, or NEXT); and far-end crosstalk (FE-XT, or FEXT). In links 100, NE-XT interference is due to signals that are transmitted in opposite directions and is illustrated by interference 103 that is caused by transmitter 102a to receiver 104b. In NE-XT interference the output of a nearby transmitter goes into a nearby receiver, and the transmitter and receiver need not be using the same technology (e.g, ADSL) to communicate. In the case of FE-XT, interference is due to signals that are transmitted in the same direction, and again, the transmitters and receiver need not be using the same technology (e.g, ADSL) to communicate. FE-XT interference is illustrated by interference 105 that is caused by transmitter 106a to receiver 104b. 
‘Self NE-XT’ refers to NE-XT interference that is due to transmitters using the same technology as the receiver. For example, ADSL modems cause interference to other ADSL modems. ‘Self FE-XT’ refers to FE-XT interference that is due to transmitters using the same technology as the receiver. Again, an example would be ADSL modems causing interference with other ADSL modems.
The nature of self-NEXT and self-FEXT is such that increasing the average transmitted power generally does not appreciably change the distance over which a modem can communicate because crosstalk power changes in direct proportion to the average transmitted power. On short loops, VDSL (very high speed digital subscriber line) modems provide high bandwidth, but have limited reach due to self-NEXT and self-FEXT. VDSL modems typically transmit data in the 13 Mbps to 55 Mbps range over distances of about 4500 feet of twisted pair copper wire, but longer ranges are also becoming possible and higher data rates may be possible as well.
Nevertheless, dynamically varying the transmit power for VDSL modems has been discussed at the International Telecommunications Union (ITU). A proposal that was made involved switching between a full power mode and no power mode and using a secondary control channel to alter modem state. The proposal suffers from at least two principal defects: 1) sudden changes from no power to full power by one or more modems are likely to result in other modems losing synchronization (i.e., not ‘environmentally friendly’ to other devices trying to communicate) and having to ‘re-train’ to regain synchronization; and 2) the switching mechanism using the secondary control channel is relatively too slow. Furthermore, regaining synchronization is a relatively slow process. Consequently, the problems of changing power quickly and minimizing the adverse effect on other devices are substantial issues that need addressing with solutions that overcome the deficiencies of the prior art.